High Energy Density Lithium-ion Batteries Research Articles

The Facile and Controllable Synthesis of Ultrafine Sn Nanocrystals Loaded on Carbon Black for High-Performance Lithium Storage.

Sn and C nanocomposites are ideal anode materials for high-energy and high-power density lithium ion batteries. However, their facile and controllable synthesis for practical applications is still a critical challenge. In this work, a facile one-step method is developed to controllably synthesize ultrafine Sn nanocrystals (< 5 nm) loaded on carbon black (Sn@C) through Na reducing SnCl4 by mechanical milling. Different from traditional up-down mechanical milling method, this method utilizes mechanical milling to trigger bottom-up reduction reaction of SnCl4. The in-situ formed Sn nanocrystals directly grow on carbon black, which results in the homogeneous composite and the size control of Sn nanocrystals. The obtained Sn@C electrode is revealed to possesses large lithium diffusion coefficient, low lithiation energy barrier and stable electrochemical property during cycle, thus showing excellent lithium storage performance with a high reversible capacity (942 mAh g-1 at a current density of 100 mA g-1), distinguished rate ability (480 mAh g-1 at 8000 mA g-1) and superb cycling performance (730 mAh g-1 at 1000 mA g-1 even after 1000 cycles).

Influence of the temperature-induced ordered-disordered phases for iron difluoride on the electrochemical performance

Iron difluoride (FeF2) is one of the promising cathodes to achieve high energy-density lithium-ion batteries (LIBs). However, the sluggish ion transportation kinetics in the FeF2 bulk phase may cause large voltage hysteresis and lower cell efficiency, which hinders its commercial adoption. Introducing a disordered phase into the ordered bulk FeF2 is proposed to promote ion transportation and increase ion diffusion channels. Here, varying ratios of the disordered phase are incorporated into the ordered matrix of FeF2 by adjusting the annealed temperatures. The ion transportation is improved due to the optimal ratio of the disordered to ordered (RDO) phase. Among four electrodes, the RDO25% electrode demonstrates the least electrochemical polarization of 1.22 V and the lowest voltage hysteresis of about 1.0 V, in addition to delivering a discharge capacity of 128 mAh g−1 after 100 cycles in the carbonate-type electrolyte. The introduction of the disordered phase enables FeF2 to exhibit a distinct intercalation and conversion dual-mechanism cathode. Additionally, appropriate ratios of the disordered-ordered phase can mitigate nanoparticle agglomeration, establish a conformal cathode electrolyte interface (CEI), reduce interfacial resistance, and thereby enhance ion transport.

Rigid-flexible mediated Co-polyimide enabling stable silicon anode in lithium-ion batteries

Silicon is considered as a promising electrode materials for the next generation of lithium-ion batteries. However, it’s commercial applications are hindered by significant volume changes. In this study, copolyimide binders with adjustable rigidity and flexibility were synthesized through simple copolymerization. The rigid segments containing adenine groups (p-APA) provide excellent mechanical strength and interface interaction, while flexible segments containing siloxane bonds and alkane chains (DS-NH2) can buffer volume changes through enhanced mobility and topological entanglement of molecular segments. Optimizing the copolymerization ratio helps minimize damage to the silicon electrode structure and stabilizes solid electrolyte interphase (SEI) growth. Compared to the rigid polyimide binders, rigid-flexible coupling strategy improves cycling stability, remaining a capacity of 2170 mAh/g at 2 A/g after 200 cycles and exhibiting stable cycling for over 750 cycles with a capacity limitation of 1500 mAh/g. The high mass loading silicon anodes, micron-sized silicon oxide electrodes and the full cells also exhibit favorable electrochemical performance. This research provides valuable insights for designing high-performance aromatic polymer binders for high energy density lithium-ion batteries.

Mitigating PTFE decomposition in ultra thick dry-processed anodes for high energy density lithium-ion batteries

Dry electrode technology is a next-generation method for manufacturing lithium-ion batteries because it is useful for fabricating thick electrodes without solvents, facilitating high energy densities and cutting down on the battery manufacturing costs. However, the commonly used polytetrafluoroethylene (PTFE) binder in dry electrode technology undergoes severe decomposition in dry-processed anodes during the first lithiation process due to its low lowest unoccupied molecular orbital level. This phenomenon seriously aggravates battery performance, such as in terms of the initial coulombic efficiency and cycle life. Thus, a strategy to suppress this irreversible reaction of PTFE should be established for dry-processed anodes to increase the energy density of LIBs without adverse effects on battery performance. To address this challenge, in this work, fluoroethylene carbonate (FEC) as an electrolyte additive has been introduced to form a preemptive and stable FEC-derived solid electrolyte interface (SEI) to protect a graphite and the PTFE binder. This SEI considerably alleviates the irreversible reaction of PTFE, thereby securing the reversible capacity and maintaining the structure of the electrode through the great binding properties. These results provide guidance for increasing the electrochemical stability in dry-processed anode systems, which gets closer the innovative dry anode technology for cost-effectiveness and high energy density.

Cut-off voltage influencing the voltage decay of single crystal lithium-rich manganese-based cathode materials in lithium-ion batteries

The voltage decay of Li-rich layered oxide cathode materials results in the deterioration of cycling performance and continuous energy loss, which seriously hinders their application in the high-energy–density lithium-ion battery (LIB) market. However, the origin of the voltage decay mechanism remains controversial due to the complex influences of transition metal (TM) migration, oxygen release, indistinguishable surface/bulk reactions and the easy intra/inter-crystalline cracking during cycling. We investigated the direct cause of voltage decay in micrometer-scale single-crystal Li1.2Mn0.54Ni0.13Co0.13O2 (SC-LNCM) cathode materials by regulating the cut-off voltage. The redox of TM and O2− ions can be precisely controlled by setting different voltage windows, while the cracking can be restrained, and surface/bulk structural evaluation can be monitored because of the large single crystal size. The results show that the voltage decay of SC-LNCM is related to the combined effect of cation rearrangement and oxygen release. Maintaining the discharge cutoff voltage at 3 V or the charging cutoff voltage at 4.5 V effectively mitigates the voltage decay, which provides a solution for suppressing the voltage decay of Li-rich and Mn-based layered oxide cathode materials. Our work provides significant insights into the origin of the voltage decay mechanism and an easily achievable strategy to restrain the voltage decay for Li-rich and Mn-based cathode materials.

Design of double layer cathode electrode for improving the safety and stability of lithium-ion batteries

LiNi0.8Co0.1Mn0.1O2 (NCM) is a widely used cathode material for lithium-ion batteries (LIBs). However, the poor cycle performance and safety issue remains a huge challenge for its practical applications. Here we show a simple double layer strategy to improve the electrochemical characteristics and safety performance. NCM was coated by LiFePO4 (LFP), a safe cathode material with long cycle stability, exhibited more excellent cycling stability and similar rate properties at room temperature (called NCM/LFP). Under 45 ℃, the NCM/LFP (6:1)||graphite pouch cell shows excellent 83% capacity retention after 600 cycles at 1C, which compared to the sharp attenuation of pristine NCM. Furthermore, NCM/LFP (6:1)||graphite cell could pass the nail penetration tests and the thermal runaway temperature (TTR) delayed by 193℃, which proved the double layer design has better safety performance. The current work might pave an avenue for cathode materials, as a sustainable solution to high energy density LIBs for applications.

Over-Lithiation Regulation of Silicon-Based Anodes for High-Energy Lithium-Ion Batteries.

Mitigating the growth of dendritic lithium (Li) metal on silicon (Si) anodes has become a crucial task for the pursuit of long-term cycling stability of high energy density Si-based lithium-ion batteries (LIBs) under fast charging or other specific conditions. While it is widely known that Li metal plating on Si-based anodes may introduce inferior cycling stability and cause safety concerns, the evolution of the anode/material structure and electrochemical performance with Li metal plating remains largely unexplored. A comprehensive quantitative investigation of the hybrid Li storage mechanism, combining the Li alloying/dealloying mechanism and plating/stripping mechanism, has been conducted to explore the effect of Li plating on Si-based anodes. The findings reveal that Li plating/stripping accounts for the decay of the overall Coulombic efficiency and cycling stability of the hybrid Li storage mechanism. Furthermore, alloying reactions occurring below 0 V encourage the formation of crystalline Li15Si4, which subsequently exacerbates voltage hysteresis. The performance decay is amplified as the ratio of Li plating/stripping capacity increases, or in other words, as the over-lithiation level rises, thereby posing a threat to the battery's cycling stability. These results provide valuable insights into the design of advanced Si-based electrodes for high energy density LIBs.

Boosting interface compatibility of high-voltage LiNi0.5Mn1.5O4 cathode materials with an ionic-conductive Li0.33La0.56TiO3 coating

High voltage LiNi0.5Mn1.5O4 (LNMO) is a crucial cathode material for lithium-ion batteries (LIBs) aiming at high energy densities. However, its performance is hindered by the dissolution of transition metal ions and the instability of the cathode electrolyte interface (CEI) film at elevated voltages. To address these issues, we report a solvothermal method to prepare LNMO coated with Li+-conductor Li0.33La0.56TiO3 (LNMO@LLTO). Our study reveals that the LLTO coating not only enhances ion diffusion at the interface, minimizing the Li+ concentration gradient, and acts as a barrier against Mn-ion dissolution. This coating minimizes side reactions, mitigates structural degradation, and promotes the formation of a stable and balanced CEI film that incorporates LLTO, thus ensuring the long-term cycle stability. When compared to bulk LNMO, the LNMO@LLTO cathode demonstrates superior performance, maintaining reversible capacities of 111.4 and 102.2 mAh·g−1 after 500 cycles at 2 and 5C at 25 °C, respectively. Even at 60 °C, the LNMO@LLTO maintains a high capacity of 94.3 mAh·g−1 with minimal electrochemical polarization after 200 cycles at 2C. This simple preparation technique, together with its performance promotion, offers new insights into the development of high voltage cathode materials for high energy density LIBs.

Borate modified Co-free LiNi0.5Mn1.5O4 cathode material: A pathway to superior interface and cycling stability in LNMO/graphite full-cells

Cobalt-free LiNi0.5Mn1.5O4 (LNMO) holds great promise as a next-generation cathode material for high-energy and high-power density lithium-ion batteries (LIBs), making it a key contender for large-scale energy storage systems (ESS) and transportation applications. Despite its potential, the commercial utilization of LNMO has been impeded by its rapid capacity deterioration attributed to an unstable LNMO/electrolyte interface caused by the intrinsically high operating voltage of LNMO. Here, we present a pragmatic and scalable strategy to improve the LNMO/electrolyte interface through borate-based surface coating of the LNMO. Optimizing the coating process yielded high-rate capability and stable LNMO/electrolyte interface analyzed by extended float tests. Borate-LNMO materials show superior cycling stability at 25 °C and 45 °C, attributed to a robust cathode electrolyte interphase (CEI) formation. The bare LNMO and 0.25-B2O3-LNMO demostrated a cycling stability of around 41.20 % and 62.50 % respectively after 1000 cycles in LNMO/graphite full-cells. Post-mortem X-ray photoelectron spectroscopy (XPS) analysis revealed fewer side reaction products on borate-LNMO cathodes compared to bare LNMO. Additionally, scanning electron microscopy (SEM) revealed a thicker and denser solid-electrolyte interphase (SEI) on cycled graphite anodes from bare LNMO cells, while a thin and robust SEI was observed on graphite from borate-LNMO cells. Finally, we have conclusively demonstrated that the primary aging mechanism in LNMO/graphite full cells stems from the instability at the LNMO/electrolyte or/and graphite/electrolyte interface, rather than material-related degradation phenomenon. These compelling outcomes underscore the potential of cathode material surface coating in general and especially of borate coating on LNMO as a promising and cost-effective enabler for the use of LNMO in next-generation LIBs.

Improving electrochemical performances of LiNi0.5Mn1.5O4 by the strategy of oxygen vacancy doping

Despite the dazzling potential to be utilized as a high energy density lithium-ion battery cathode, the practical application of LiNi0.5Mn1.5O4 (LNMO) is hampered by the structural distortion related to Jahn-Teller effect of Mn3+ and the capacity degradation related to Mn2+ dissolution at high voltage. Herein, LNMO cathode materials doped with different contents of oxygen vacancies are synthesized by directly adjusting the oxygen ratio in the calcination atmosphere. Experimental and theoretical calculation results show that doping an appropriate amount of oxygen vacancies can regulate the electronic structure of Mn and Ni around oxygen vacancy, adjust the relative amount of Mn3+/Mn4+, increase the degree of disorder, and improve the electronic and ionic conductivity of LNMO, thereby enhancing its electrochemical performance. Especially, the modified LNMO exhibits a high capacity of 121.2 mAh g−1 at 0.5 C, a good capacity retention of 87.6 % after 100 cycles, and an excellent rate performance when the content of oxygen vacancies is about 20.60 %, corresponding to the oxygen ratio of 50 % in the calcination atmosphere. This study emphasizes the influence of doping oxygen vacancies on LNMO performance, enhancing its electrochemical properties without the introduction of additional elements. Moreover, these findings offer a fresh approach to enhancing the electrochemical performance of advanced batteries.

Understanding the limitations of thick electrodes on the rate capability of high-energy density lithium-ion batteries

As a core component of transportation electrification, lithium-ion batteries need to address two critical challenges: achieving high energy density and enabling fast charging. However, changes in electrode thickness could result in a trade-off between these two characteristics. Herein, the impact of electrode thickness on the electrochemical performance, thermal behavior, as well as energy and power density of LiNi0.8Co0.1Mn0.1O2(NCM)/graphite batteries are investigated through mathematical modeling. Underutilization of active materials of the thick electrode is identified as the primary limitation on rate capability, which can be attributed to the slow solid-state diffusion and poor electrolyte transport within the electrode, especially the enlarged concentration gradients in the solid phase and the lithium depletion in the electrolyte phase during high-rate charging. Evolutions of local current density and heat generation rate along the electrode thickness are compared, identifying the underlying mechanism of inducing uneven reaction kinetics inside the battery. The trade-off between the energy and power density as a function of electrode thickness is investigated based on the limitations of charge transfer and mass transport. Furthermore, the rate-limiting factors under various application requirements are elucidated. This study will provide guidance for the electrode design of high-energy-density batteries with fast-charging capabilities.

Enhanced cycling stability in 4.6 V LiCoO2 for high energy density lithium-ion batteries through Al and Y co-doping

For the first time, this work investigates the synergetic impact of yttrium (Y) and aluminium (Al) co-doping on the electrochemical properties of 4.6 V LiCoO2 (LCO) cathode for high energy density lithium-ion batteries. The optimized LCO demonstrates an impressive initial discharge capacity of 208.6 mAh g−1 in the voltage range of 3–4.6 V, along with superior structural stability, retaining over 86 % of its initial capacity even after 125 cycles at the current of 100 mA g−1 that is far higher than the capacity retention of 40 % for the pristine LCO under the same condition. Thanks to the synergistic effect, Al in Co layers enhances the bonds of Al/Co-O to stabilize oxygen in the LCO crystal structure while Y serves “pillar” in Li layers, migrated from the initial Co layers during cycling process, effectively curtail the formation and expansion of interlayer cracking, which contributes to the long-term cycle stability of LCO under high voltage. Most importantly, Al/Y co-doping is a facile process compatible with the existing industrial infrastructure for LCO mass production, and thus it can be scaled up readily for commercial applications. Al/Y co-doped LCO is assembled into commercial-level pouch cells with graphite anode, achieves an extremely high energy density of 750 Wh/L, and exhibits a superior cycling stability and obtains a capacity retention of 88.2 % at 25 °C after 800 cycles with the high cutoff voltage of 4.55 V.

Noncombustible gel polymer electrolyte inspired by bio-radical chemistry for high voltage and high safety Ni-rich lithium batteries

For high energy density lithium-ion batteries (LIBs) with nickel-rich ternary cathodes, the chemical degradation of electrolytes caused by free radical reactions and the hazards of thermal runaway have always been significant challenges. Inspired by the free radical scavenging of living organisms and multiphase synergistic flame retardant mechanism, we innovatively designed and prepared a multifunctional flame retardant HCCP-TMP that combines flame retardancy and free radical scavenging by combining hindered amine and cyclophosphazene. Only 1 wt% HCCP-TMP can make the polyacrylate-based gel polymer electrolyte (GPE) incombustible. Moreover, the equipped NCM811//Graphite pouch cells don't exhibit combustion behavior after thermal runaway and can resist mechanical abuse. Based on the above noncombustible GPE, the NCM811//Li battery exhibits capacity retention rate of 82.2 % after 100 cycles at a current density of 2 C and in the voltage range of 3.0–4.7 V, exhibiting excellent cyclability under high voltage. This simple molecular design simultaneously improves the fire safety and high voltage stability, demonstrating enormous application potential in the field of advanced LIBs with high safety and high energy density.

Carbon/C3N4 heterostructures constructed from lignin toward enhanced lithium-ion storage

Lithium-ion batteries (LIBs) are widely used in portable energy storage. The capacity of commercial graphite is difficult to improve due to the stoichiometry limit of LiC6 of graphite, thus new anodes need to be developed to meet the demand of high-energy–density LIB. The growing interest in graphitized carbon nitride (g-C3N4) stems from its structural resemblance to graphite and its capacity to offer abundant adsorption and intercalation sites. However, g-C3N4, as a semiconductor, has a low lithium transfer rate due to its poor conductivity and high diffusion resistance. Improving the electron transport rate of g-C3N4 and reducing the adsorption energy barrier of Li+ in g-C3N4 are the keys to improving the electrochemical performances of g-C3N4. In this study, lignin and melamine were homogeneously mixed using the spray drying method, followed by the preparation of covalently bonded C3N4/LC material through a one-step carbonization process. The uniform dispersion of g-C3N4 in amorphous carbon can improve the conductivity and reduce the diffusion energy barrier of Li+. As a result, the C3N4/LC-x anode has better electrochemical behavior, including higher reversible capacity, better rate performance, and cycle stability.

Composition and state prediction of lithium-ion cathode via convolutional neural network trained on scanning electron microscopy images

High-throughput materials research is strongly required to accelerate the development of safe and high energy-density lithium-ion battery (LIB) applicable to electric vehicle and energy storage system. The artificial intelligence, including machine learning with neural networks such as Boltzmann neural networks and convolutional neural networks (CNN), is a powerful tool to explore next-generation electrode materials and functional additives. In this paper, we develop a prediction model that classifies the major composition (e.g., 333, 523, 622, and 811) and different states (e.g., pristine, pre-cycled, and 100 times cycled) of various Li(Ni, Co, Mn)O2 (NCM) cathodes via CNN trained on scanning electron microscopy (SEM) images. Based on those results, our trained CNN model shows a high accuracy of 99.6% where the number of test set is 3840. In addition, the model can be applied to the case of untrained SEM data of NCM cathodes with functional electrolyte additives.

Layer by Layer: Improved Tortuosity in High Loading Aqueous NMC811 Electrodes

Thick electrode production is a key enabler for realizing high energy density Lithium-ion batteries. Therefore, the investigation of tortuosity as a crucial limiting parameter was conducted in this work. A thickness threshold (>150 μm) for a drastic increase in tortuosity for aqueous processed LiNi0.8Mn0.1Co0.1O2 (NMC811) electrodes was determined. Symmetrical cells, under blocking conditions, were analyzed via electrochemical impedance spectroscopy. To counteract this phenomenon, multi-layer coated electrodes with varying binder concentrations were investigated. This novel coating method, combined with the reduction of binder material, leads to a tortuosity decrease of more than 80%, when compared to high-loading electrodes (>8.5 mAhcm−2) coated with the conventional doctor-blade technique. Additionally, a simplified transmission line model is opposed to a linear fitting method for analyzing the impedance data.

Unveiling the vacancy-mediated charge kinetics and capacity fading mechanism in Mn-based cathode for lithium-ion battery

In the recent past, Li2MnSiO4 (LMS) has been considered a promising cathode material for high energy density lithium-ion batteries (LIBs). Extraction of the maximum amount of Li from the host LMS and reversibility of Li insertion and extraction is still challenging, probably because of poor conductivity and, thereby, poor Li-kinetics. To improve the Li-diffusion and ionic conductivity, oxygen vacancies (OVs) in pristine LMS (LMS180) have been created, and then their impact on the Li-reactivity of LMS is studied. The quantity of OVs is determined by X-ray photoelectron spectroscopy (XPS) and calculated to be 6.8% in LMS180 heated at 300 ºC in Ar (LMSA300) and ∼ 8.2% in LMS180 heated at 500° C in Ar (LMSA500), subsequently it is supported and validate by the electron paramagnetic response (EPR) and high-resolution transmission electron microscopy (HR-TEM). The best sample, i.e., LMSA500, exhibits a total conductivity of 8.06×10−6 S/cm, resulting in an initial charge capacity of 292 (±5) mAh/g at 10 mA/g. The OVs are found to facilitate the facile lithium-ion movement by providing the lowest migration energy. Furthermore, knowing the impacts of OVs presence in LMS and their effect on Li-diffusion enables us to address traditional capacity fading processes in LMS.

Regulation of surface structure to suppress voltage decay for high-stable Li-rich oxide cathodes

Layered lithium-rich manganese oxides (LMO) are promising cathode materials for high-energy density lithium-ion batteries due to their high specific capacity and working potential. However, serious voltage decay and undesirable cycle stability are big obstacles that hinder their industrialization and application. Herein, an innovative and industrially valuable strategy of constructing composite coatings and oxygen vacancies on the LMO surface is put forward to suppress voltage decay and enhance the cycling stability of LMO by precisely modulating NaBH4 treatment. The synergistic effect of surface spinel coating, NaBO2coating, and oxygen vacancies effectively inhibits the release of oxygen, impedes the occurrence of side reactions at the interface, inhibits the migration and dissolution of transition metals, suppresses the irreversible structure transformation and improves the reversibility of redox reaction. The capacity retention of the NaBH4-treated LMO material maintains 100 % after 200 cycles at 0.5C, as well as stable cycling at 2C with a discharge capacity of 180 mAh g−1 harvested. Moreover, the voltage decay is significantly suppressed by NaBH4 modification, especially the NaBH4-0.5 % sample exhibits a mere voltage decay of 1.73 mV per cycle in the 200 cycles. This study provides significant insights into the development of LMO cathodes with long-cycle performance.

Development and understanding of the lithiation/de-lithiation mechanism of a low cobalt and nickel-rich cathode material for lithium‐ion batteries

Nickel-rich LiNixMnyCo1−x−yO2 (NMC, with x ≥ 0.6) layered oxides are regarded as cathode candidates for high-energy density lithium-ion batteries (LIBs). However, this technology faces several challenges due to the scarcity of crucial raw materials such as cobalt precursors and the negative environmental impact of their extraction process. It is therefore, it becomes imperative to develop low cobalt content cathode compositions that offer alternative options with comparable electrochemical performance. Herein, we have used a systematic approach to modify the composition while keeping all other properties (particle size and morphology) constant to synthesise low-Co-cathode materials, namely LiNi0·8Mn0·1Co0·1O2 (NMC-10 %) and LiNi0·8Mn0·15Co0·05O2 (NMC-5%). A co-precipitation route using a continuous stirred thank reactor was used for synthesis. The physico-chemical properties, electrochemical performances, cycling stability, and rate capabilities of NMC-5% materials are comparable to those of NCM-10 % materials. NMC-10 % and NMC-5% half cells show initial specific discharge capacities of 191 mAh.g−1, and 185 mAh.g−1, respectively, in a voltage range of 2.8–4.3 V (vs. Li+/Li). The NMC-5% exhibits excellent thermal stability, characterized by a predominant exothermic peak at 432 °C and significantly lower total heat energy when compared to NMC-10 % counterpart, which displayed its main exothermic peak at 339 °C. The structural and electronic changes of both materials were evaluated through in operando synchrotron-based X-ray techniques. Regardless of the cobalt content, both materials show comparable structural and electronic evolution upon the de-intercalation/intercalation of Li+ ions. An irreversible phase transition at the first delithiation was observed as emphasized by diffraction of synchrotron radiations.

Homogeneously dispersed silicon/graphite composite toward enhanced lithium-ion batteries

At present, silicon/graphite composite is the most promising anode material for high energy density lithium-ion batteries. However, during the preparation, nanoscale silicon will inevitably undergo agglomeration, which will cause serious volume changes and breakage during the charging/discharging process. How to achieve uniform dispersion of nanoscale silicon onto the surface of graphite becomes a great challenge. Here, by grafting functional groups on the surface of silicon, both the repulsive force between silicon particles and the binding force between silicon/graphite have been improved, thereby highly uniform dispersed silicon is achieved. After being coated with asphalt and etching off the functional groups, the conductivity and ion migration ability of the composite are further improved. Resultantly, the modified composite provides a high discharging capacity of 598.4 mAh·g−1 at 0.2C and can still maintain a high-capacity retention of 90.7 % and a small volume expansion rate of 30.8 % after 500 cycles.